The present invention concerns apparatus for generating electric current. It is particularly concerned with providing a compact and reliable generator which is readily portable. Such a generator can find many fields of application one of which would be for use in providing power for portable radios, computers and other electronic equipment in environments where there is little or no provision of electrical mains.
It is of course well known to provide generators driven by peddle power. These require continuous effort in order to be operational. Additionally a radio having a removable spring-power generator has been disclosed in UK Patent Specification No 2262324. A more sophisticated form of a mechanically powered generator is disclosed in European Patent Specification No EP-A-96925832.6. As in the previous specification the main embodiment is applied to a portable radio.
The systems disclosed in these two patent specifications have a number of disadvantages. Firstly when switched off the generator is electronically braked. However this does not provide a full lock to the system and spring energy is slowly bled away through the system and lost.
A more important drawback is that a user of the system is unable to re-energise the spring, if this is the source of mechanical energy, whilst maintaining the requisite output current. Thus if the system is used in a radio and the spring winds down a user cannot re-energise the spring system and listen to the radio at the same time as winding the spring disengages any energy released to the generator and power delivery stops for the duration of the wind up.
Another disadvantage is that the energy capacity is limited to the size of the spring fitted. This provides for a limited play back period when fully charged and should extended play times be required larger heavier and more expensive springs need to be fitted. Therefore relatively large and expensive springs have to be used in order to provide acceptable play times.
Finally, the spring, gear box, and generator combination specifically disclosed in the above two specifications incurs quite heavy energy losses. The spring releases energy at a fixed rate equivalent to the maximum energy demanded by the radio, or any other load which may be applied. Energy less than that is shunted and thus wasted. Thus the spring unwind duration is the same as the play time of the radio and typically will be between 30 minutes and 1 hour. Frictional losses in the transmission are roughly constant and are not dependent on the unwind velocity so that any attempt to increase unwind time has a consequent energy penalty. From this it follows that the longer the unwind time of the spring the higher the energy content that is lost during that unwind cycle.
International Patent Specification No. WO98/42060 discloses a portable telephone set having a battery self-charging module having a manual device for driving a generator to charge a battery cell.
The present invention has as one concern to alleviate the above mentioned problems.
In accordance with the present invention there is provided a power system for supplying a variable load, the system comprising means for mounting a rechargeable battery; a mechanical generator for generating electrical power; a source of mechanical energy for driving the generator, output terminal means for connection to a load, and a control circuit for controlling the supply of electric power to the output terminal means, and wherein the mounting means and the generator are connected in parallel to an output rail, whereby output energy of said mechanical generator in excess of a load requirement is used to charge a battery mounted in said mounting means the output rail being connectable to said output terminal means by a mechanical switch which in its open position prevents conduction through the rail and by a switching transistor located between the switch and an output terminal of the output terminal means, the arrangement being such that closure of the switch enables a pulse to turn on the switching transistor to allow passage of electrical current to the output terminal, maintenance of the switching transistor in its on condition being dependent upon the output voltage supplied to the output terminal means being above a predetermined threshold.